Liquid electrophotographic developing compositions

ABSTRACT

An electrostatic liquid developer for the development of electrostatic charge patterns is provided wherein the said developer comprises an electrically insulating carrier liquid having a dielectric constant of less than 3 and a volume resistivity in excess of 109 Ohm.cm, resin-coated toner particles suspended in said carrier liquid and an alkylated polymer of a heterocyclic N-vinyl monomer. By the presence of the said polymer excellent developers of positive polarity can be obtained.

United States Patent Gilliams et al.

1451 Sept. 30, 1975 [54] LIQUID ELECTROPHOTOGRAPI-IIC DEVELOPINGCOMPOSITIONS [75] Inventors: Yvan Karel Gilliams, Berchcm;

Jozef Leonard Van Engeland, St. Katelijne-Waver; Noe'l Jozef De Volder,Edegem, all of Belgium [73] Assignee: Agfa-Gevaert N.V., Mortsel,Belgium [22] Filed: June 28, 1973 [21] Appl. No.: 374,774

[30] Foreign Application Priority Data July 12, 1972 United Kingdom32655/72 [52] US. Cl. 252/62.1; 117/37 LE; 96/1 LY [51] Int. Cl. G03G9/00 [58] Field of Search 252/62.1 L; 117/37 LE [56] References CitedUNITED STATES PATENTS 3,432,329 3/1969 Van Den Heuvel et al. 252/621 L3.542.682 ll/l970 Mutaffis 252/621 L 3,585,140 6/1971 Machida et all252/621 L 3,657,130 4/1972 Machida et al. 252/621 L PrimaryE.\'uminerMaynard R. Wilbur Assistant E.\'umilzer-T. M. Blum Attorney,Agent, or Firm-A. W. Breiner resin-coated toner particles suspended insaid carrier liquid and an alkylated polyrner of a heterocyclic N- vinylmonomer. By the presence of the said polymer excellent developers ofpositive polarity can be obtained.

38 Claims, No Drawings merit of electrostatic charge velopers usedtherefor.

LIQUID ELECTROPI-IOTOGRAPI-IIC" DEVELOPING COMPOSITIONS I The presentinvention relates to electrophotography, and more particularly toimprovements in the developpatterns and to liquid dc- Knownelectrophotographic processes comprise the 'steps'of electrostaticallycharging in the dark a photoconductive surface, image-wise exposing thesaid surface whereby the irradiated areas become discharged inaccordance with the intensity of radiation, thus forming a latentelectrostatic image and developing the material to form a visible imageby depositing on the image a finely divided 'electroscopic materialknown as toner. The thus developed image may be fixed to the surface ofthe photoconductor or transferred to another surface and fixed thereon.Instead of forming the latent electrostatic image by the steps describedabove it is also possible to directly charge the photoconductive layerin image configuration.

In one type of electrophotography, the latent electrostatic image isdeveloped or rendered visible by use of liquid developers comprisingcoloured toner particles suspended in an insulating carrier liquid,which should have a volume resistivity in excess of IO Ohm.cm and adielectric constant below 3. The suspended toner particles, usuallyfinely divided pigments or dyes, are elec- I trostatically charged anddevelop the latent image under influence of the charge of the latentelectrostatic image.

It is known to produce positive-working and negative-working liquiddevelopers. A negative-working developer contains toner particles whichare attracted by positive electrostatic charges, or which upon contactwith a surface bearing latent image areas formed by negativeelectrostatic charges are repelled by such negative charges and depositonto the non-image areas. A positive working developer behaves in thereverse sense and thus contains particles which are attracted bynegative electrostatic charges. 7

Electrophotographic coatings containing zinc oxide are generally chargednegatively with the result that the latent image formed by exposure isnegative in polarity.

.Thus, a positive developer can be applied to a zinc oxide layercontaining a negative latent image to produce a facsimile reproductionof the original radiation patternwhereas a negative developer can beapplied to such layer to produce areversal reproduction of the originalradiation pattern.

The polarity of the toner particles with respect to the electrostaticlatent image to be developed is determined by the nature of thematerials used in the preparation of the liquid developer and byso-called polarity control agents, which confer to the suspended tonerparticles either a negative or positive charge.

According to U.S. Pat. No. 3,542,681 alkylated polymers of heterocyclicN-vinyl monomers induce in liquid to'ner compositions strong negativepolarity on the suspended toner particles. These polymeric materials arepreferably materialsobtained by alkylation of homopolymers or copolymersof a N-vinyl lactam monomer, most preferably a N-vinyl pyrrolidonemonomer with an a-olefin containing from 2 to 2000 carbon atoms, orobtained by simultaneously polymerizing and alkylating a mixturecontaining a heterocyclic N-vinyl monomer and an a-olefin.

Surprisingly it has now been found that these alkylated polymers ofheterocyclic N-vinyl monomers give excellentdevelopers of positivepolarity when the toner particles are suspended in an insulating liquidand the said toner particles are overcoated with a resin.

The present invention thus provides a positiveworking electrostaticliquid developing composition comprising an electrically insulatingcarrier liquid having a dielectric constant of less than 3 and a volumeresistivity in excess of 10 Ohm.cm, resin-coated toner particlessuspended in said carrier liquid and an alkylated polymer of aheterocyclic N-vinyl monomer.

Resin-coated toner particles are widely known in the art of liquidelectrostatic developers. The resin functions as fixing agent to fix thetoner particles where they are deposited e'lectrostatically. Theseresins.include natural as well as synthetic resins. Examples of resinsthat can be used in accordance with the present invention are esters ofhydrogenated rosin and longoil, rosin-modified phenol formaldehyderesins, pentaerythritol esters of rosin, glycerol esters of hydrogenatedrosin, short oil and medium oil alkyd resins, polyvinyl esters e.g.polyvinyl stearate, coumarone-indene resins, polyacrylates andpolymethacrylates, e.g. polyisobutylmethacrylate, polystyrene,styrenebutadiene copolymers, cyclized rubber, etc. Preferred inaccordance with the present invention are coumarone-indene resins e.g.PICCOVAR 440 (trade name of the Picco Company USA) cyclized rubber e.g.ALPEX 514 of Reinhold Albert Chemie A.G., W. Germany as well as rosinand synthetic resins derived from rosin e. g. the PENTALYN, POLYPALEESTER, and STAYBE- LITE ESTER resins (registered trade marks ofHercules) such as PENTALYN A, a'pentaerythritol ester of wood rosin withacid No. 12, PENTALYN H, a pentaerythritol ester of hydrogenated woodrosin with acid No. 13, PENTALYN X, a pentaerythritol ester ofhydrogenated wood rosin with acid No. 14, POLYPALE ESTER 10, a glycerinester of polymerized wood rosin with acid No. 6 and STAYBELITE ESTER 10,a glycerin ester of hydrogenated wood rosin with acid NO. 8; AMBEROL M21and M82 (registered trade marks of the Rohm & Haas Company), which arewood rosinmodified phenol formaldehyde resins. Other specific examplesof suitable resins are POLYSTYRENE PS 3 marketed by Dow, LUVISKOL VA 64of BASF, which is a co(vinylpyrrolidon-vinylacetate) (:40), POLYS- TYROLB of BASF, which is a co(styrenebutylacrylate), KYRAX A of AIRKO, USA,which is a polyvinyl stearate.

From the prior art it is known that these fixing agents need notnecessarily be present as a coating'over the toner particles and may beincluded in the liquid composition as a separate addition. It is pointedout that when such fixing agents are added separately to the liquidcomposition and thus not coated on the pigment particles beforehand, theabove alkylated polymers of heterocyclic N-vinyl monomers provide anegativeworking toner composition or a toner composition with bothnegative and positive characteristics, which is therefore unsuitable foruse. Accordingly, it is surprising that an exceptionallypositive-working toner composition can be provided with the abovealkylated polymers of heterocyclic N-vinyl monomers when the tonerparticles are resin-coated.

The positive polarity increases with the concentration of the saidalkylated polymers so that the polarity can be easily controlled and anoptimum relationship obtained. However, exception must be made forresincoated carbon blacks .where the amount of said a1kyl ated polymersmust be limited to at most by weight relative to the amountofresin-coated carbon black since'otherwise the polarity may becomereversed. As is set forth hereinafter, this maximum percentage can behigher by the presence in the developing composition of other positivelypolarizing agents and/orthe use of a mixture of carbon black and acolour-improving dyestuff as colouring matter that is resin-coatedResin-coated toner particles are particularly advantageous inelectrophotographic multicolour reproduction processes. In theseprocesses images are produced by repeating for each colour separationimage the successive operations of-charging the photoconductive element,exposing to a colour image through a filter absorbing the colour to bereproduced and developing by means of toner particles of the desiredcolour. In order to allow recharging of the photoconductive element forthe formation of the subsequent colour separation image, the image areasof the toner first deposited should be capable-of carrying theappropriate charge and comprise'therefore an insulating material e.g. aresin coated over the, toner deposit. It is possible forthis purpose toincorporate the insulating resins in the liquid developing composition.However, large amounts are required which reduces the intensity .ofcolour deposit. Improved results are obtained when in the liquiddeveloping composition, the toner particles are overcoated with theresin. In accordance with the present invention, the resin-coated tonerparticles allow proper recharging of the photoconductive element.

The alkylated polymers of heterocyclic N-vinyl monomers which are usedin accordance with the present invention in combination withresin-coated toner particles are preferably derived: fron N-vinyllactammonomers, mostpreferably from N-vinylpyrrolidone and N Pat. No.3,542,681, which is incorporated herein by reference; the alkylatedpolymers of the heterocyclic N-vinyl monomers suitable for use inaccordance with the present invention can be prepared by alkylation of ahomopolymer or copolymer of such heterocyclic N- vinyl monomer with analpha-olefin containing from 2 carbon atoms to 2,000 carbon atoms,preferably in the range of from 2 to 200 carbon atoms, and mostpreferably in the range offrom 8 to 42 carbon atoms, said alkylationprocess beingmore fully described in U.S. Pat. No. 3,417,054, or bysimultaneously polymerizing and alkylating a mixture containing aheterocyclic N-vinyl monomer and an alpha-olefin containing from 2carbon atoms to 2,000 carbon atoms or a mixture of the same or. twodifferent heterocyclic N-vinyl monomers and an alpha-olefin containingfrom 2 carbon atoms to 2,000

carbon atoms, said simultaneous polymerization and alkylation processbeing more fully described in U.S. Pat. No. 3,423,367.

Representative examples of heterocyclic N-vinyl monomers, the alkylatedpolymer derivatives of which can be used in accordance with the presentinvention can be found in the above U.S. Pat. No. 3,542,681 which alsoincludes representative examples of monoethylenically unsaturatedpolymerizable monomers suitable for copolymerization with the saidheterocyclic N-vinyl monomers to form copolymers which are readilyalkylated as described above. More details as regards the a-olefinsemployed for producing the alkylated polymers can also be found in thesaid U.S. Pat. No. 3,542,681. It can also be learned from this U.S.Patent that the alkylated polymers have a K-value comprised between 10and 140, preferably between 30 and 100. Particularly suitable for use inaccordance with the present invention are GANEX 216, GANEX 220 andANTARON V216 which are olefin-alkylated poly-N-vinylpyrrolidone with 20vinylpyrrolidone residue commercially available from GAF.

By means of the alkylated polymers of the above heterocyclic N-vinylmonomers, the positive polarity of the resin-coated toner particles canbe easily controlled. This makes possible to obtain an optimumrelationship, as a function of the electrostatic latent image charge,between the developable density and the stability of the tonercomposition against settling of the toner particles. Moreover, it wasfound that the positiveworking toner compositions of the presentinvention are perfectly suitable for continuous-tone multicolourreproduction in that the gradation is dependent on the polarity of thetoner particles; the more positive the polarity the softer the gradationthat is obtained.

In addition to the alkylated polymer of the heterocyclic N-vinylmonomers, a liquid'electrophotographic developer according to thepresent invention can contain other positively polarizing agents, e.g.metal soaps of fatty acids such as aluminium stearate, zinc stearate,lead stearate, copper stearate, cadmium stearate, calcium stearate, zincpalmitate, aluminium palmitate, lithium octoate, aluminium dresinate(aluminium salt of abietic acid), vanadium dresinate, tin dresinate,copper linoleate, manganese linoleate, etc. and the bivalent ortrivalent metal salts of an oxyacid derived from phosphorus containingat least one organic residue according to British Pat. No. 1,151,141.

As is described in the British Pat. No. 1,151,141, the bivalent ortrivalent metal salts of a phosphorus oxyacid are preferably bivalent ortrivalent metal salts of:

a. a monoester or diester of an oxyacid derived from phosphorus,

b. an oxyacid derived from phosphorus and containing one or two organicgroups linked to the phosphorus atom by a carbon atom, or

c. an oxyacid derived from phosphorus and containing an ester group andan organic group linked by a carbon atom to the phosphorus atom.

The said organic group can be aliphatic, cycloaliphatic or aromatic.

The salt comprises an organic residue e.g. organic group(s) and/or estergroup(s) rendering the salt substantially soluble in the electricallyinsulating carrier liquid. The organic residue preferably comprises achain of at least 4 carbon atoms, most preferably from 10 to 18 carbonatoms, and such chain may be substituted and/or interrupted byheteroatom(s), e.g. oxygen, sulphur, or nitrogen atom(s).

The solubility in the electrically insulating carrier liquid of themetal salts can be promoted by the presence of one or more organicgroups with branched structure, e.g. branched aliphatic groups, such asa 2-butyl-octyl group.

Excellent results are obtained where the metal of the metal salts of thephosphorus oxyacid is zinc. However,

other salts may also be used for example magnesium salts, calcium salts,strontium salts, barium salts, iron salts, cobalt salts, nickel salts,copper salts, cadmium salts, aluminium salts, and lead salts.

More details about these bivalent or trivalent metal salts,representative examples thereof, and methods of preparing these saltscanbe found in the aboveBritish Patent, which is incorporated herein byreference.

By the use of the above supplemental positively polarizing agents it wasfound possible to obtain a liquid developing composition having apositive polarity exceeding that of the composition, with the alkylatedpoly-N-vinyl heterocycle alonefParticularly favourable results could beobtained when in addition to the alkylated poly-N-vinyl heterocycle, thezinc salt of mono-=2 butyl-octyl phosphate or aluminium dresinate isused.

When such supplemental positively polarizing agent is used in developingcompositionscomprising resincoated carbon blacks the above limitation ofmaximum 10 ,7! by weight of alkylated heterocyclic N-vinyl polymer nolonger stands.

The liquid developer composition of the present invention comprises as abase fluid any of the conventional electrically insulating carrierliquids generally employed in liquid developer compositions. Thus, forexample the carrier liquid includes various hydrocarbon solvents; e.g.aromatic hydrocarbons such as benzene, toluene and xylenes, aliphatichydrocarbons such as hexane, cyclohexane and heptane; fluorocarbons andsilicone oils. The carrier liquid is preferably a commercial petroleumdistillate e.g. mixtures of aliphatic hydrocarbons preferably havingaboiling point comprised between 150C and 220C such as the IS OPARS G, H,K and L (trade names) of the Esso Standard Oil Company, SHELL SOL T(trade name) of the Shell Oil Company, etc. In view of its lowdielectric constant and high resistivity the carrier liquid isnon-polar,

The, pigment or colouring agent used as toner may be any of the pigmentsand dyestuffs commonlyemployed in liquid electrostatic tonercompositions providedthat they carry a resin-coating. Thus, forexample,the colouring agent or pigment can be carbon black and various analogousforms thereof e.g. lamp black, channel black and furnace black. In viewof the above limitation of the maximum amount of alkylated heterocyclicN- vinyl polymer, it is preferred to use in developing com positionscontaining resin-coated carbon blacks, one of the above other positivelypolarizing agents in addition to the alkylated heterocyclic N-vinylpolymer. It is also possible to use a mixture of carbon black and a bluecolour-improving dyestuff as toner that isresin-coated, in which case itis also possible to increase the amount of alkylated heterocyclicN-vinyl polymer beyond the above limit of 10 /z.

Suitable coloured pigments include azo dyes, xanthene dyes,phthalocyanine dyes, which may be in X- form e.g. as described inpublished German Patent (DOS) 1,944,021, triphenylmethane dyes,diphenylmethane dyes, stilbene dyes, acridine dyes, quinoline dyes,quinone imine dyes, thiazine dyes, azine dyes, etc.

A large variety. of dyes are known for use in liquid electrophotographicdevelopers and therefore, the following dyes are given for illustrationpurposes only: Heliogen Blau LG (a metal free phthalocyanine bluepigment of BASF), Monastral Blue (acopperiphth alocyanine pigment Cl74,160), Heliogen Blau B pulver (BASF), l-lelioecht Blau HG (Bayer),.Brillia nt Carr'nine 6B' (Cl115,850), Helioecht Rose (Bayer),Quinacridone Magenta (CI. Pigment Red 122), Thioindigo Magenta (e.g.C173,31 Fanalrosa B supra pulver (BASF), Lithol Rubine (BASF), lrgalithecht Gelb (Geigy), Benzidin Yellow (C.l. 21,090, 21,100), Hansa Yellow(e.g. Cl 11680), Permanentgelb GR 52 (Hoechst),-Monolit GelbGNS (1C1),Helioecht Gelb GRN (Bayer), etc.

.As is pointed out above, the pigments or dyes used in accordancewiththe present invention should be resincoated, which can be achievedby any suitable means. This may be effected e.g. by. mixing the pigmentor dye with the resin, heating the vmixture to melting, and kneading themelt. The kneaded mass is then cooled, broken, and ground to finepowder.

It is alsopossible to use commercially available resincoated dyes orpigments-e.g. those marketed by Ciba- Geigy for example Microlith CT,which is carbon black coated with a resinderived from rosin, andMicrolithe Blue 4GT,,Microlithe Blue 22027T, Microlithe Yellow mixing.When the resin is merely added as a separate additive, the: alkylatedheterocyclic N-vinyl polymer has no favourable polarizing effect. 7

The mannerin which the liquid composition is produced is by no waycritical. As is illustrated by the Examplesnhereinafter, it isconventional to prepare by means ofsuitahle mixers, e.g. 3-rollmills,ball mills, colloid mills, homogenizers, high speed stirrers, etcl aconcentrate in the insulating carrier liquid of the materials selectedfor the composition, and subsequently adding further insulating carrierliquid to provide the liquid toner composition ready for use in theelectrostatic reproductionprocess.

The resin-coated pigment or dye is employed in the composition ready foruse in the amount necessary to be capable of being deposited, whenattracted to the electrostatic image,-so that the desired image densityis reached. Ingeneral, .the pigment or colouring agent is used in anamount comprised between about 0.01 g and 10 g per litre, preferablybetween about 0.01 g and about 2 g per litre.

The ratio of resin to pigment'or dye may vary within .very wide limitsThe resin preferably outweighs the pigment or dye but should permit toobtain sufficient density. A preferred weight ratio is comprised betweenabout 1:1 andaboutlzlO. l

The amount of the positively polarizing alkylated polymer of aheterocyclic N-vinyl monomer as well as of other possible positivelypolarizing agents e.g. the metal soaps and bivalent or trivalent metalsalts of phosphorus oxyacids as referred to. hereinbefore, .will varywith the nature of the charge already on the resincoated toner particlesand depends on the desiredieffect e.g. obtaining a developer yieldingimagesof steep or soft gradation. Moreover, as is noted above it ispossible to control by means of the amount of the above 2GT, MicrolitheYellow 22018T, Microlithe Bordeaux positively polarizing agents, thepolarity of the toner particles so as to obtain optimum relation betweenimage density and contrast and stability of the liquid compositionagainst settling of the toner particles. An individual determination ofthe required quantity may be made for each new combination by somesimple experiments. The amounts should, of course, be such that they donot lower the volume resistivity of the resulting composition below 10Ohm.cm or raise the dielectric constant above 3.

The following examples illustrate the present invention.

EXAMPLE 1 200 g of STAYBELITE ESTER l (Hercules) and 100 g of HelioechtBlau HG (Bayer) were mixed at 6065C in a mixing and kneading apparatusof MEILI, Switzerland, type liliput O30 L.N. The kneaded mass was cooledand ground in a grinding apparatus l.K.A., model A (of Janke & Kunkel,W. Germany) to obtain a fine powder of resin-coated pigment.

In a ball-mill 2 g of the above resin-coated pigment powder were groundfor 10 hours together with:

l 1.5 g of a 25 76 by weight solution of NEOCRYL B 702 (trade name for acopolymer of butylmethacrylate and stearyl methacrylate comprising about1 7c of methacrylic acid marketed by Polyvinylchemie, The Netherlands)in ISOPAR G (trade name for an aliphatic hydrocarbon solvent having aboiling range of l60l75C and a KB value of 27, marketed by the E550Standard Oil Company) 0.5 g of a 25 7c by weight solution of ANTARONV2l6 marketed by GAF in ISOPAR G 2 ml of a 0.2 (g/vol) solution inISOPAR G of the zinc salt of mono-2-butyloctyl phosphate,

25 ml of ISOPAR G.

When 10 ml of the above concentrated liquid developing composition wasdiluted with 1 litre of ISOPAR G a stable positive-workingelectrophotographic liquid developer was formed for the development of alatent electrostatic image formed on a conventional electrophotographicrecording element comprising paper coated with photoconductive zincoxide in a resinous binder by negative charging and image-wise exposureto light. The developer was suitable for continuoustone reproduction.

Increasing amounts of ANTARON V 216 or of the above zinc salt improvethe stability of the developer but at the cost of a reduction of thedevelopable maximum density. For a given density region, however, theoptimum stability can be determined.

EXAMPLE 3 Example I was repeated with the only difference that thepigment now used was Fanalrosa B supra pulver (BASF).

A stable positive-working electrophotographic liquid developer wasformed suitable for continuous tone reproduction.

EXAMPLE 4 EXample l was repeated with the only difference that thepigment now used was carbon black (PRIN- TEX G marketed by Degussa A.G.,W.-Germany) and that 0.1 g of the solution of ANTARON V2 l 6 was usedand 10 ml of the zinc salt solution.

A very good positive-working developer was obtained.

EXAMPLE 5 Example 1 was repeated with the only difference that in thepreparation of the resin-coated pigment, the 200 g of STAYBELITE ESTERl0 (Hercules) was replaced by 200 g of POLYPALE ESTER l0 (Hercules).

EXAMPLE 6 Example I was repeated with the only difference that in thepreparation of the resin-coated pigment, the 200 g of STAYBELITE ESTERl0 (Hercules) was replaced by 200 g of PENTALYN A (Hercules).

EXAMPLE 7 Example I was repeated with the only difference that in thepreparation of the resin-coated pigment, the 200 g of STAYBELITE ESTER10 was replaced by 200 g of a coumarone-indene resin marketed by thePICCO Comp, USA, under the trade name PICCOVAR 440 and that in thepreparation of the developer concentrate l g of the ANTARON V 216solution was used.

An ideal positive-working electrophotographic liquid developer wasobtained suitable for the reproduction of continuous tone images.

EXAMPLE 8 Example 1 was repeated with the only difference that in thepreparation of the resin-coated pigment, part of the STAYBELITE ESTER 10was replaced by PICCO' VAR 440.

When replacing 40 g, g or g of the STAYBE- LITE ESTER 10 by 40 g, 100 gor 160 g respectively of PICCOVAR 440 and further using the resin-coatedpigment obtained as described in example 1 for the preparation of anelectrophotographic liquid developer, positive-working developers wereobtained suitable for continuous tone reproduction.

EXAMPLE 9 g of STAYBELITE ESTER l0 (Hercules), 20 g of the aluminiumsalt of abietic acid and 100 g of Helioecht Blau HG (Bayer) were mixedand molten together as described in example I. The melt was thorouglykneaded and then cooled whereupon it was ground to obtain a fine powderof resin-coated pigment.

The following ingredients were ground in a ball-mill for 12 hours.

10 g of a 30 7c by weight solution of NEOCRYL B 702 in ISOPAR G 0.5 g ofa 25 70 by Weight solution of ANTARON V216 in ISOPAR G 2 g of the aboveresin-coated pigment powder, and

35 ml of ISOPAR G.

When 10 ml of the above concentrated liquid developing composition wasdiluted with 1 litre of ISOPAR G a very satisfactory positive-workingelectrophotographic liquid developer was formed suitable forcontinuous-tone reproduction.

EXAMPLE 10 form a positiveworking electrophotographic liquid developergiving high density images when used for the development ofelectrostatic charge patterns.

EXAMPLE 11 Example 10 was repeated with the difference that in thepreparation of the developer concentrate 6 g of the NEOCRYL B 702solution and 6 g of the ANTARON V 216 solution were used. 1

A highly stable positive-working electrophotographic liquid developerwas obtained suitable for continuous tone reproduction. The maximumdensity obtainable was slightly lower than with the developer of exampleb 10.

EXAMPLE 12 Example 10 was repeated with the difference that in thepreparation of the developer concentrate 1 1.5 g of the NEOCRYL B 702solution and 0.5 g of the AN- TARON V 216 solution were used and thatalso 4 ml of a 0.2 7: (g/vol) solution in ISOPAR G of the zinc salt ofmono-2-butyloctylphosphate was added.

The positive-working electrophotographic liquid developer obtained upondilution had high-stability and yielded upon use for the development ofelectrostatic charge patterns images of favourable density.

EXAMPLE 13 Example 10 was repeated using MlCROLlTl-l CT (a resincoatedcarbon black of CIBA, the said resin being STAYBELITE ESTER 10) aspigment.

A positive-working liquid electrostatic developer suitable forcontinuous-tone reproduction was obtained.

EXAMPLE 14 Example 10 was repeated using MlCROLlTl-l Gelb 2GT (aresin-coated yellow pigment of CIBA) as pigment.

A positive-working liquid electrostatic developer suitable forcontinuous-tone reproduction was obtained.

EXAMPLE 15 The liquid electrostatic developer compositions ofexamples 1. 2 and 3 can be used successfully for the reproduction ofmulticolour images.

A panchromatically sensitized photoconductive zinc oxide element havinga paper support was negatively charged (about 400 V) and exposed througha blue filter (e.g. a Kodak Wratten filter 473 or an Agfa- Gevaert bandfilter V449) to a multicolour photographic silver halide transparencyfitted in a conventional photographic enlarger. The selectivelydischarged photoconductive layer was developed by means of 'the yellowelectrostatic liquid developer of example 2' to form the yellowseparation image.

The photoconductive element carrying the yellow toner image was thenrecharged either in dry or wet condition and again exposed (in register)to the transparency, now through a green filter (e.g. a Kodak Wrattenfilter 61 or an Agfa-Gevaert band filter U531 The selectively dischargedpip toconductive layer was developed by means of the magentaelectrostatic liquid developer of example 3 to form the magentaseparation image.

The steps of charging and exposure were then repeated using now a redfilter (e.g. a Kodak Wratten filter 29 or an Agfa-Gevaert cut-off filterL622). Development occurred by means of the cyan electrostatic liquiddeveloper of example 1 to form the cyan separation image.

The ultimate result was a very good continuous tone reproduction of thetransparency.

The sequence according to which the separation images are formed byexposure and corresponding development can be altered at will.

What we claim is:

1. A positive-working electrostatic liquid developer which will developelectrostatic charge patterns comprising an electrically insulatingcarrier liquid having a dielectric constant of less than 3 and a volumeresistivity in excess of 10 Ohm.cm, resin-coated toner particlessuspended in said carrier liquid, and an alkylated polymer of aheterocyclic N-vinyl monomer.

2. A liquid developer according to claim 1 wherein said alkylatedpolymer of a heterocyclic N-vinyl monomer is an alkylated polymer of aN-vinyl lactam monomer.

3. A liquid developer according to claim 2, wherein said N-vinyl lactammonomer is a N-vinyl pyrrolidone or a N-vinylpiperidone.

4. A liquid developer according to claim 1 wherein said alkylatedpolymer is formed by alkylation with an alpha-olefin.

5. A liquid developer according to claim 4 wherein said alpha-olefincomprises from 8 to 42 carbon atoms.

6. A liquid developer according to claim 1 wherein the toner particlescarry a coating of a synthetic resin.

7. A liquid developer according to claim 1 wherein the toner particlescarry a coating of a natural resin.

8. A liquid developer according to claim 7 wherein said resin is rosinor a resin derived from rosin.

9. A liquid developer according to claim 7 wherein said resin is acoumarone-indene resin or cyclized rubber.

10. A liquid developer according to claim 8 wherein said resin derivedfrom rosin is an ester of rosin or hydrogenated rosin.

1 1. A liquid developer according to claim 10 wherein said ester is apentaerythritol ester or glycerol ester.

12. A liquid developer according to claim 1 wherein the resin-coatedtoner particles are resin-coated dyes.

13. A liquid developer according to claim 1 which also comprises a metalsoap of a fatty acid.

14. A liquid developer according to claim 13. wherein said metal soap isthe aluminium salt of abietic acid.

15. A liquid developer according to claim 1, which also comprises abivalent or trivalent metal salt of oxyacid derived from phosphoruscontaining at least one organic residue.

16. A liquid developer according to claim wherein said oxyacid derivedfrom phosphorus containing at least one'organic residue is a monoesteror diester of an oxyacid derived from phosphorus.

17. A liquid developer according to claim 16, wherein said oxyacidderived from phosphorus containing at least one organic residue is amono-2-butyloctyl phosphate.

18. A liquid developer according to claim 15 wherein said metal salt isa zinc salt.

19. A liquid developer according to claim 1 wherein the carrier liquidis an aliphatic hydrocarbon.

20. Method of preparing an electrostatic liquid developer for thedevelopment of electrostatic charge patterns by mixing toner particleswith an electrically insulating carrier liquid having a dielectricconstant of less than 3 and a volume resistivity in excess of 10 ohm.cm,wherein mixing occurs in the presence of an alkylated polymer of aheterocyclic N-vinyl monomer and the said toner particles are coatedwith a resin prior to mixing was to form a positive-workingelectrostatic liquid developer.

21. Method according to claim 20, wherein said alkylatedv polymer of aheterocyclic N-vinyl monomer is an alkylated'poly'mer of a N-vinyllactam monomer.

22. Method according to claim 21, wherein said N- vinyl lactammonomer isa N-vinyl pyrrolidone or a N- vinylpiperidone.

23. Method according to claim 20, wherein said alkylated polymer isformed by alkylation with an alphaolefin.

24. Method according to claim 23, wherein said alpha-olefin comprisesfrom 8 to 42 carbon atoms.

25. Method according to claim 20, wherein the said resin is a syntheticresin.

26. Method according to claim 20, wherein the said resin is a naturalresin.

27. Method according to claim 26, wherein said resin is rosin or a resinderived from rosin.

28. Method according to claim 26 wherein said resin isa'coumarone-indene resin or cyclized rubber.

29. Method according to claim 27, wherein said resin derived from rosinis an ester of rosin or hydrogenated rosin.

30. Method according to claim 29, wherein said ester is apentaerythritol ester or glycerol ester.

31. Method according to claim 20 wherein the toner is a dye.

32. Method according to claim 20, wherein mixing also occurs in thepresence of a metal soap of a fatty acid.

33. Method according to claim 32 wherein said metal soap is thealuminium salt of abietic acid.

34. Method according to claim 20, wherein mixing also occurs in thepresence of a bivalent or trivalent metalsalt of an oxyacid derived fromphosphorus containing at least one organic residue.

35. Method according to claim 34, wherein said oxyacid derived fromphosphorus containing at least one organic residue is a monoester ordiester of an oxyacid derived from phosphorus.

36. Method according to claim 34, wherein said oxyacid derived fromphosphorus containing at least one organic residue is amono-2-butylocytl phosphate.

37. Method according to claim 34, wherein said metal salt is a zincsalt.

38. Method according to claim 20, wherein the carrier liquid is analiphatic hydrocarbon.

1. A POSITIVE-WORKING ELECTROSTATIC LIQUID EDVELOPE WHICH WILL DEVELOPELECTROSTATIC CHARGE PATTERMS COMPRISING EN ELECTRICALLY INSULATINGCARRIER LIQUID HAVING A DIELECTRIC CONSTANT OF LESS THAN 3 AND A VOLUMERESISTIVITY IN EXCESS OF 109 OHNM.CM, RESIN-COATED TONER PARICLESSUSPENDED IN SAID CARRIER LIQUID, AND AN ALKYLATED POLYMER OF AHETHEROCYCLIC N-VINYL MONOMER.
 2. A liquid developer according to claim1 wherein said alkylated polymer of a heterocyclic N-vinyl monomer is analkylated polymer of a N-vinyl lactam monomer.
 3. A liquid developeraccording to claim 2, wherein said N-vinyl lactam monomer is a N-vinylpyrrolidone or a N-vinylpiperidone.
 4. A liquid developer according toclaim 1 wherein said alkylated polymer is formed by alkylation with analpha-olefin.
 5. A liquid developer according to claim 4 wherein saidalpha-olefin comprises from 8 to 42 carbon atoms.
 6. A liquid developeraccording to claim 1 wherein the toner particles carry a coating of asynthetic resin.
 7. A liquid developer according to claim 1 wherein thetoner particles carry a coating of a natural resin.
 8. A liquiddeveloper according to claim 7 wherein said resin is rosin or a resinderived from rosin.
 9. A liquid developer according to claim 7 whereinsaid resin is a coumarone-indene resin or cyclized rubber.
 10. A liquiddeveloper according to claim 8 wherein said resin derived from rosin isan ester of rosin or hydrogenated rosin.
 11. A liquid developeraccording to claim 10 wherein said ester is a pentaerythritol ester orglycerol ester.
 12. A liquid developer according to claim 1 wherein theresin-coated toner particles are resin-coated dyes.
 13. A liquiddeveloper according to claim 1 which also comprises a metal soap of afatty acid.
 14. A liquid developer according to claim 13, wherein saidmetal soap is the aluminium salt of abietic acid.
 15. A liquid developeraccording to claim 1, which also comprises a bivalent or trivalent metalsalt of oxyacid derived from phosphorus containing at least one organicresidue.
 16. A liquid developer according to claim 15 wherein saidoxyacid derived from phosphorus containing at least one organic residueis a monoester or diester of an oxyacid derived from phosphorus.
 17. Aliquid developer according to claim 16, wherein said oxyacid derivedfrom phosphorus containing at least one organic residue is amono-2-butyloctyl phosphate.
 18. A liquid developer according to claim15 wherein said metal salt is a zinc salt.
 19. A liquid developeraccording to claim 1 wherein the carrier liquid is an aliphatichydrocarbon.
 20. Method of preparing an electrostatic liquid developerfor the development of electrostatic charge patterns by mixing tonerparticles with an electrically insulating carrier liquid having adielectric constant of less than 3 and a volume resistivity in excess of109 ohm.cm, wherein mixing occurs in the presence of an alkylatedpolymer of a heterocyclic N-vinyl monomer and the said toner particlesare coated with a resin prior to mixing so as to form a positive-workingelectrostatic liquid developer.
 21. Method according to claim 20,wherein said alkylated polymer of a heterocyclic N-vinyl monomer is analkylated polymer of a N-vinyl laCtam monomer.
 22. Method according toclaim 21, wherein said N-vinyl lactam monomer is a N-vinyl pyrrolidoneor a N-vinylpiperidone.
 23. Method according to claim 20, wherein saidalkylated polymer is formed by alkylation with an alpha-olefin. 24.Method according to claim 23, wherein said alpha-olefin comprises from 8to 42 carbon atoms.
 25. Method according to claim 20, wherein the saidresin is a synthetic resin.
 26. Method according to claim 20, whereinthe said resin is a natural resin.
 27. Method according to claim 26,wherein said resin is rosin or a resin derived from rosin.
 28. Methodaccording to claim 26, wherein said resin is a coumarone-indene resin orcyclized rubber.
 29. Method according to claim 27, wherein said resinderived from rosin is an ester of rosin or hydrogenated rosin. 30.Method according to claim 29, wherein said ester is a pentaerythritolester or glycerol ester.
 31. Method according to claim 20 wherein thetoner is a dye.
 32. Method according to claim 20, wherein mixing alsooccurs in the presence of a metal soap of a fatty acid.
 33. Methodaccording to claim 32 wherein said metal soap is the aluminium salt ofabietic acid.
 34. Method according to claim 20, wherein mixing alsooccurs in the presence of a bivalent or trivalent metal salt of anoxyacid derived from phosphorus containing at least one organic residue.35. Method according to claim 34, wherein said oxyacid derived fromphosphorus containing at least one organic residue is a monoester ordiester of an oxyacid derived from phosphorus.
 36. Method according toclaim 34, wherein said oxyacid derived from phosphorus containing atleast one organic residue is a mono-2-butylocytl phosphate.
 37. Methodaccording to claim 34, wherein said metal salt is a zinc salt. 38.Method according to claim 20, wherein the carrier liquid is an aliphatichydrocarbon.